I think he might have jumped out of the car because he didn't want to be killed by the hitch hiker. His parents who would be astonished at this sudden reaction would probably get out of the car to find out what happened to him. This then enables Jacob and his entire family be saved.

Most importantly, he would have tried to get himself as well as his family to safety. However, if the situation is less tense, I think he would have tried in one way or another, make his parents understand that Ian Rellik was strange and a dangerous man.